Tracing control method

ABSTRACT

A tracing control system machines a workpiece through tracing by repeating a plurality of tracing passes in a tracing direction which is constant to a pick feed while detecting amounts of displacement of respective axes applied to a stylus. A first abrupt change region (P1c) on a model (1) surface is detected using the amounts of displacement of the respective axes obtained in the previous tracing pass, and a first position (X1c, Y1c) of the first abrupt change region in the X-Y plane is stored. Similarly, a second abrupt change region (P2c) is detected and a second position (X2c, Y2c) in the X-Y plane is stored. The tracing speed is lowered in the next tracing pass in the section within the range of each predetermined distance (l1, l2) in front of and behind an intersection (X3p, Y3p) with an extended line passing through the first position (X1c, Y1c) and the second position (X2c, Y2c). When an abrupt change region is continued, this assumed position matches the position of an actual abrupt change region more accurately than before, and a smaller deceleration section can be set and the overall tracing time shortened.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tracing control method, and moreparticularly, to a tracing control method by which an overshot machiningof a cutter into a workpiece at a region of a model where the shapethereof undergoes an abrupt change is prevented, whereby the overalltracing time can be shortened.

2. Description of the Related Art

In a tracing control, the amounts of displacement of respective axesapplied to a stylus are detected by a tracer head, speed commands of therespective axes are calculated in a tracing calculating circuit by usingthese amounts of displacement, motors of the respective axes are drivenin accordance therewith, and the tracer head is moved along the surfaceof a model while the cutter is moved relatively against a workpiece atthe same speed, and by repeating this motion the workpiece is machinedto the same shape as the model.

Nevertheless, such a tracing control has a problem in that an overshotmachining might occur at regions of the model where the shape isabruptly changed if the tracing speed is too high.

To solve this problem, the applicants filed Japanese Patent ApplicationNo. 1-192383 wherein the title of the invention is "TRACING CONTROLMETHOD", on Jul. 25, 1990. In this Patent Application No. 1-192383, aregion in which an abrupt change of the model surface occurs is detectedbased on the amounts of displacement of the respective axes obtained bythe tracer head, the position in the tracing direction is stored, andthe tracing speed is lowered only in a section within the range ofpredetermined distances in front of and behind the position in thetracing direction stored this time, in the next tracing pass, to preventan overshot machining and accordingly, to shorten the overall tracingtime.

Nevertheless, because the position of an actual abrupt change region inthe next tracing pass is not always at the same position as that of theabrupt change region detected in this tracing pass, the range of thedeceleration section can not be set to a value small enough to cope withthis anomaly, and thus the desired shortening of the tracing time cannotbe attained.

SUMMARY OF THE INVENTION

The present invention has been made in view of the aforesaid drawbacks,and an object of the present invention is to provide a tracing controlmethod in which an overshot machining of the cutter into a workpiece ata region of a model where the shape thereof undergoes an abrupt changeis prevented, and thus the overall tracing time can be furthershortened.

To achieve the above object, in accordance with the present inventionthere is provided a tracing control method for machining a workpiecethrough tracing by repeating a plurality of tracing passes in thetracing direction which is constant to a pick feed while detectingamounts of displacement of respective axes applied to a stylus,comprising detecting a first abrupt change region on the above modelsurface using the amounts of displacement of the respective axesobtained in the previous tracing pass, storing at least a first positionin a specific plane on the above first abrupt change region, detecting asecond abrupt change region contiguous to the above first abrupt changeregion on the above model surface using the amounts of displacement ofthe respective axes obtained in this tracing pass, storing a secondposition in the above specific plane on the above second abrupt changeregion, and lowering the tracing speed in the next tracing pass in asection within the range of predetermined distances in front of andbehind an intersection of a position in the above specific plane with anextended line passing through the above first position and the abovesecond position.

Assuming that a next abrupt change region exists on the extended lineconnecting the abrupt change region of the previous tracing pass to theabrupt change region of this tracing pass, the tracing speed is loweredonly in the section in the vicinity of this assumed abrupt change regionin the next tracing pass. If the abrupt change region is continued, thisassumed position matches the position of an actual abrupt change regionmore accurately than before, and thus the deceleration section can beset to a smaller value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 and FIG. 2 are explanatory views of a tracing control methodaccording the present invention;

FIG. 3 is an explanatory view of a detection method of an abrupt changeregion according to the present invention;

FIG. 4 is a flow chart of detection processing of an abrupt changeregion in an embodiment according to the present invention;

FIGS. 5(a) and 5(b) are flow charts of a tracing control method of anembodiment according to the present invention; and

FIG. 6 is a block diagram showing constitution of a tracing controlsystem to carry out the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will hereinafter be describedwith reference to the drawings.

FIG. 1 is an explanatory view of a tracing control method according tothe present invention. In FIG. 1, a tracer head 3 is moved relatively inthe X-axis direction against a model 1 while being moved verticallyalong the shape of the model 1 in the Z-axis direction, bringing astylus 2 into contact with a surface of the model 1 from a startingpoint P1a, and a section of a pass 11 is traced at a relatively lowtracing speed F1.

Here, changes in the contour of the model 1 are determined by a method,described later, using amounts of displacement of the respective axesdetected by the tracer head 3 at predetermined sampling times duringthis tracing, and a point P1c of an abrupt change region is detectedthereby. When the point P1c is detected, the coordinate values (x1c,y1c) on the X-Y plane thereof are stored.

When tracing to a point P1e is finished, the stylus 2 is removed fromthe model 1 and moved to a point P0 in the vicinity of the point P1a,further moved to a point P2a by carrying out a pick feed 21 in theY-axis direction, and during tracing of a pass 12 carried out from thispoint at the speed F1, a point P2c of an abrupt change region isdetected as before and the coordinate values (x2c, y2c) thereof on theX-Y plane are stored.

When the tracing to a point P2c is completed, the stylus 2 is removedfrom the model 1 and returned to the point P2a, and then moved to apoint P3a by carrying out a pick feed 22 in the Y-axis direction, andthe tracing of a pass 13 is started from this point at a desirable quicktracing speed Ff. When the position on the pass 13xy on the X-Y planereaches a point P3b of coordinate values (x3b, y3b) by a distance l1 infront of intersection coordinates (x3p, y3p) at which an extended lineL12 passes through the coordinate values (x1c, y1c) and (x2c, y2c), thetracing speed is immediately lowered to Fs, and an abrupt change regionP3c is passed at this speed. Then, after reaching a point P3d ofcoordinate values (x3d, y3d) ahead of the intersection coordinates (x3p,y3p) by a distance l2, the tracing speed is raised to Ff and tracing iscontinued to a point P3e.

Note, the contour change is also determined by using the amounts ofdisplacement from the tracer head 3 in this pass 13, whereby the pointP3c of the actual abrupt change region in the pass 13 is detected andthe coordinate values thereof on the X-Y plane are stored. Thereafter,tracing is started at the tracing speed Ff in the next pass, and whenthe position in the X-Y plane enters the range within the distances l1and l2 respectively in front of and behind the intersection at which anextended line passes through coordinate values (x2c, y2c) and coordinatevalues of an abrupt change region newly stored during the pass 13, thetracing speed is lowered to Fs and increased again when the tracingmoves outside of this range.

As mentioned above, the tracing is carried out by detecting regions atwhich the shape of the model undergoes an abrupt change, whereby thetracing speed is lowered only in a section in front of and behind anintersection of the position on the X-Y plane at which an extended linepasses through each of the coordinate values of abrupt change regionsdetected in the previous pass and the pass before the previous pass, andthus the tracing can be carried out at desired high speed in othersections.

Where a plurality of abrupt change regions exist in the course of asingle pass, it is necessary to specify from which abrupt change regionin the previous pass does the abrupt change region detected in this passcontinue.

Such a case is shown in FIG. 2. When tracing a model 10 in this Figure,X and Y coordinate values of abrupt change regions P4c and P4f detectedin a previous pass 14, and displacement directions of the stylus D4c andD4f in each abrupt change region, are stored, the displacementdirections of the stylus D4c and D4f are compared with a displacementdirection of the stylus D5c at a point of an abrupt change region P5cwhen detected in this pass 15, and the abrupt change region P4c inalmost the same direction is determined to be an abrupt change regioncontiguous to the abrupt change region P5c.

Next, a method of detecting an abrupt change region will be explainedwith reference to FIG. 3. In this Figure, vectors Ex, Ey and Ez arevectors in the direction the X-, Y- and Z-axes respectively and having asize corresponding to the amounts of displacement of the respective axesεx, εy and εz detected by the tracer head 3, and E is a synthetic vectorthereof. That is:

    |E|=(εx.sup.2 +εy.sup.2 +εz.sup.2).sup.1/2.

The angle θ made by the vector E in the X-Y plane and the angle φ madeby the vector E with the Z-axis are calculated by the followingequations:

    θ=tan.sup.-1 (εy/εx)                 (1)

    φ=cos.sup.-1 (εz/ε)                    (2).

Here, ε is the size of the vector E, i.e., a synthetic amount ofdisplacement, and thus the angles θ and φ are independent of each otherin the three-dimensional coordinate system in FIG. 3, and a contourchange of a three-dimensional model can be easily recognized bymonitoring these changes.

At this stage, the values of the angles θ and φ at the previous samplingare stored, the differences therebetween and each of the values obtainedat this sampling are calculated, and an abrupt change region isdetermined when a total value of these differences exceed a presetvalue.

Also, the above mentioned displacement direction of the styluscorresponds to the direction of the vector E, and whether or not this isa contiguous abrupt change region is determined therefrom.

FIG. 4 is a flow chart of a detection processing of an abrupt changeregion in an embodiment according to the present invention. In thisFigure, numerical values following the letter S show the number of thestep.

[S1] The amounts of displacement εx, εy and εz detected by the tracerhead are sampled at predetermined sampling times.

[S2] The angles θ1 and φ1 at this sampling are calculated by the aboveequations (1) and (2).

[S3] The difference between the angle θ1 at this sampling and the angleθ0 at the previous sampling and the difference between the angle φ1 atthis sampling and the angle φ0 at the previous sampling are calculated,and the total value ω is obtained.

[S4] It is determined whether or not the total value ω is larger than aset value Ω. If it is larger, the process goes to S5, and if it is notlarger, the process is finished.

[S5] An abrupt change region is determined, and the X and Y coordinatesat the present position are stored.

FIGS. 5(a) and 5(b) are flow charts of a tracing control method of anembodiment according to the present invention.

[S11] If this is a tracing pass for a first or a second time, theprocess goes to S12, but if this is a tracing pass made after a thirdtime, the process goes to S13.

[S12] Tracing is started at a tracing speed F1.

[S13] Tracing is started at a desired high tracing speed Ff.

[S14] It is determined whether or not an abrupt change region exists,according to the flow chart in FIG. 4. If an abrupt change regionexists, the process goes to S15, and if it does not, the process goes toS19.

[S15] The X and Y coordinates at the present position and displacementdirection of the stylus are stored.

[S16] It is determined whether or not this abrupt change region iscontiguous to the abrupt change region of the previous pass, i.e., it isdetermined whether or not the displacement direction of the stylusmatches the displacement direction in the abrupt change region of theprevious pass. If there is a match, the abrupt change region isconsidered to be contiguous to that of the previous pass, and theprocess goes to S17. If there is no match, the abrupt change region isconsidered not to be contiguous to the previous pass (including thefirst and the second passes), and thus the process goes to S18.

[S17] In the X-Y plane, coordinate values of an intersection of the nextpass with an extended line passing through the abrupt change region ofthe previous pass and the abrupt change region of this pass contiguoustherefrom are obtained, and accordingly, the position in front of thisposition by the distance l1 is set as a deceleration start position andthe position following this position by the distance l2 is set as adeceleration end position.

[S18] Coordinate values on the next pass transversely shifted againstthe present position by 90°, i.e., in the Y-axis direction, areobtained, and the position in front of this position by the distance l1is set as a deceleration start position and the position following thisposition by the distance l2 is set as a deceleration end position.

[S19] If the tracing head is at the deceleration start position, theprocess goes to S20, and if not, the process goes to S23.

[S20] The tracing speed is lowered to Fs.

[S21] If the tracing head is at the deceleration end position, theprocess goes to S20, and if not, tracing is continued at the tracingspeed Fs.

[S22] The tracing speed is increased to Ff.

[S23] If the tracing head is at the end of the pass, the process goes toS24, and if the tracing head is not at the end of the pass, the processreturns to S11 to continue the tracing of this pass.

[S24] If the tracing is not finished, the process goes to S25.

[S25] A pick feed is carried out and a tracing of the next pass isstarted.

FIG. 6 is a block diagram showing the constitution of a tracing controlsystem according to the present invention. In FIG. 6, a processor 31reads a system program stored in a ROM 32 through a bus 39, and controlsthe overall operation of a tracing control system 30 according to thesystem program. A RAM 33 stores the amounts of displacement of therespective axes detected by the tracer head 3 and other temporary data.A nonvolatile memory 34 is powered by a battery (not shown) and storesvarious parameters such as tracing direction and tracing speed, etc.input from a control panel 40 through an interface 35.

The tracer head 3 provided in a tracing machine 50 detects the amountsof displacement εx, εy and εz in the X, Y and Z directions generated bybringing the tip end of the stylus 2 into contact with the model 1, andinputs these amounts to the processor 31.

The processor 31 generates a speed command Vx for the X-axis, a speedcommand Vy for the Y-axis and a speed command Vz in the Z-axis based onthe amounts of displacement εx, εy and εz and a commanded tracingdirection and tracing speed according to the known process. These speedcommands are analog converted by D/A converters 36x, 36y and 36z, inputto servo amplifiers 37x, 37y and 37z and output thereby, and servomotors52x, 52y and 52z of the tracing control machine 50 are driven inaccordance therewith.

Accordingly, the tracer head 3 is moved in the Z-axis direction so thatthe relative position between the tracer head 3 and the model 1 is keptconstant, and at the same time, a table 51 is moved in the Z-axisdirection and the Y-axis direction at right angles to the paper surfaceto thereby machine a workpiece 55 to the same shape as the model 1 by acutter 54, which is Z-axis controlled in the same way as the tracer head3.

Also, pulse coders 53x, 53y and 53z are provided in these servo motors52x, 52y and 52z and generate detection pulses FPx, FPy and FPz atpredetermined rotations of the servomotors. Present position registers38x, 38y and 38z in the tracing control system 30 obtain presentposition data Xa, Ya and Za of the tracer head 3 by counting up/down thedetection pulses FPx, FPy and FPz according to the respective rotationdirection, and the position data is read by the processor 31 to therebyenable a control operation such as an approach and pick feed of thetracer head 3.

Note, the tracing direction in the above explanation is not limited tothe X-axis but can be set for other axes or to a direction at a certainangle to these axes. Also, the present invention is not limited totracing of a one-way surface but can be applied to other tracingprocesses of repeating a plurality of passes in the tracing directionwith a constant pick feed, such as reciprocal surface tracing andcontour tracing.

According to the present invention, as described above, because theposition of an abrupt change region in the next tracing pass is assumedby using displacement amounts from the tracing head during tracing, andthe tracing speed is lowered in front of the assumed abrupt changeregion by only a predetermined distance in the next tracing pass,tracing at a desired high tracing speed in regions other than thissection will not cause the cutter to overshoot a workpiece. Further,since this abrupt change region is determined, for example, in a planeat right angles to the tracing plane, to be an intersection of anextended line connecting the abrupt change region of the previous passto the abrupt change region of this pass, an error in the actual abruptchange region is small, and thus a smaller deceleration section can beset and the overall tracing time shortened.

Also, as it is determined whether or not the abrupt change region iscontiguous by storing the displacement direction of the stylus in theabrupt change region in the previous pass, and by comparing thisdirection with the displacement direction of the stylus in the abruptchange region in this pass, the present invention can be applied to amodel having a plurality of abrupt change regions existing in the courseof a single pass, whereby the applicability of the present invention iswidened.

What is claimed is:
 1. A tracing control method for machining aworkpiece through tracing a model surface by repeating a plurality oftracing passes in a tracing direction at a tracing speed with shiftingby a pick feed between each pass, while detecting amounts ofdisplacement of a stylus along respective axes, comprising the stepsof:(a) detecting a first abrupt change region on said model surfaceusing the amounts of displacement of said stylus along the respectiveaxes obtained in a first tracing pass and storing at least a firstposition in a specific plane corresponding to said first abrupt changeregion; (b) detecting a second abrupt change region contiguous with saidfirst abrupt change region on said model surface using the amounts ofdisplacement of said stylus along the respective axes obtained in asecond tracing pass and storing a second position in said specific planecorresponding to said second abrupt change region; and (c) lowering thetracing speed in a third tracing pass in a section within a range ofpredetermined distances in front of and behind a point on said modelsurface in the third tracing pass and corresponding to a position insaid specific plane on an extended line passing through said firstposition and said second position.
 2. A tracing control method accordingto claim 1,wherein said detecting in step (a) further includes storingwith said first position a displacement direction of said stylus in saidfirst abrupt change region, wherein said detecting in step (b) detectsthe second abrupt change region contiguous with said first abrupt changeregion when the displacement direction of said stylus in a predeterminedabrupt change region detected in the second tracing pass matches thedisplacement direction stored with said first position, within apredetermined tolerance.
 3. A tracing control method according to claim1, wherein said specific plane is a plane at a right angle to a tracingplane.
 4. A tracing control method according to claim 1, wherein theamounts of displacement of said stylus along the respective axes aresampled at predetermined sampling times,wherein said detecting in steps(a) and (b) comprises the substeps of:(1) obtaining a plurality ofvectors corresponding to the amounts of displacement of said stylusalong the respective axes; (2) calculating an angle between saidplurality of vectors; (3) calculating a difference between the anglecalculated in substep (2) during consecutive passes; and (4) detectingone of said first and second abrupt change regions based on saiddifference calculated in substep (3).
 5. A tracing control methodaccording to claim 4,wherein said calculating in substep (2) produces aplurality of angles which are independent of each other between saidplurality of vectors, wherein said calculating in substep (3) includescalculating differences between said plurality of angles at eachsampling and obtaining a sum thereof, and wherein said detecting the oneof said first and second abrupt change regions when the sum of saiddifferences exceeds a set value.